Network topology is the study of the arrangement or mapping of the elements (links, nodes, etc.) of a network, especially the physical (real) and logical (virtual) interconnections between nodes. A local area network (LAN) is one example of a network that exhibits both a physical topology and a logical topology. Any given node in the LAN will have one or more links to one or more other nodes in the network and the mapping of these links and nodes onto a graph results in a geometrical shape that determines the physical topology of the network. Likewise, the mapping of the flow of data between the nodes in the network determines the logical topology of the network.
Thus, network topology describes the specific physical or logical arrangement of the elements of a network. The elements may be physical or logical such that physical elements are real, and logical elements may be, for example virtual elements or an arrangement of the elements of a network. Two networks may share a similar topology if the connection configuration is the same, although the networks may differ in other aspects such as physical interconnections, domains, distances between nodes, transmission rates, and/or signal types. A network may incorporate multiple smaller networks. By way of example, a private telephone exchange is a network and that network is part of a local telephone exchange. The local exchange is part of a larger network of telephones which permit international calls, and is networked with cellular telephone networks.
Any particular network topology is determined only by the graphical mapping of the configuration of physical and/or logical connections between nodes. LAN Network Topology is, therefore, technically a part of graph theory. Distances between nodes, physical interconnections, transmission rates, and/or signal types may differ in two networks and yet their topologies may be identical. The arrangement or mapping of the elements of a network gives rise to certain basic topologies which may then be combined to form more complex topologies (hybrid topologies). The most common of these basic types of topologies include bus (such as Linear, Distributed Bus), star, ring, mesh (including a partially connected or a fully connected mesh), tree, hybrid that is composed of one or more network topologies, and point-to-Point.
Logical topology corresponds to a mapping of the apparent connections between the nodes of a network, as evidenced by the path that data appears to take when traveling between the nodes. The logical classification of network topologies generally follows the same classifications as those in the physical classifications of network topologies, the path that the data takes between nodes being used to determine the topology as opposed to the actual physical connections being used to determine the topology. Logical topologies are often closely associated with media access control (MAC) methods and protocols. The logical topologies are generally determined by network protocols as opposed to being determined by the physical layout of cables, wires, and network devices or by the flow of the electrical signals, although in many cases the paths that the electrical signals take between nodes may closely match the logical flow of data, hence the convention of using the terms ‘logical topology’ and ‘signal topology’ interchangeably. Logical topologies are typically able to be dynamically reconfigured by special types of equipment such as routers and switches.
Layer-2 topology mapping is difficult to accomplish because of the way Layer-2 switching data is organized within switches. A switch maintains a table of MAC addresses organized by port where each port has one or more MAC address entries for every MAC address received on that port. In the simplest example, a port in the table will have a single MAC address which can be used to uniquely map a network node that corresponds to that single MAC address to the given switch port. However, even this simplest case may not represent the true topology as switch tables can contain out-of-date or incomplete data depending on the network traffic that flows through the switch.
Further complicating Layer-2 topology mapping is the port data associated with links between switches. For example, when one switch, e.g. T1, is directly connected to another switch, e.g. T2 on port 3, the table maintained in switch T2 is likely to have many MAC address entries for port 3. This storage of multiple MAC address entries for port 3 is because some or all of the MAC addresses known by switch T1 that transmit data through switch T2 will be present in switch T2, port 3. It is these intra-switch links that present the most difficult challenge in rendering accurate network topology maps.
In conventional methodologies, STP data is regularly transmitted between switches for two general purposes: (a) to identify and prevent loops in network topology and (b) to select the fastest routes between switches when redundant switch links are present. Switches that implement STP maintain a table of directly-connected neighbor switches based on receipt of these periodic data. These STP table data can be used to identify intra-switch links and thereby reduce the complexity in rendering intra-switch connections and identifying and rendering other Layer-2 network connections.